Suppression of GHS-R in AgRP Neurons Mitigates Diet-Induced Obesity by Activating Thermogenesis

Simultaneous treatment with palm-LEAP2(1-14) and feeding high-fat diet attenuates liver lipid metabolism but not obesity: Sign of selective resistance to palm-LEAP2(1-14)

Liver-enriched antimicrobial peptide 2 (LEAP2) is a natural antagonist/inverse agonist of ghrelin receptor GHSR. Its truncated palmitoylated analog palm-LEAP2(1-14) promised anti-obesity properties because it exhibited favourable stability and an acute anorexigenic effect in our previous studies. Here we demonstrate desirable palm-LEAP2(1-14) pharmacokinetics, with significant levels of the peptide persisting in mouse blood 3 h after its subcutaneous administration. Palm-LEAP2 (1-14) reduced ghrelin-induced c-Fos immunoreactivity in arcuate nucleus and area postrema, in line with previously described silencing of ghrelin orexigenic effect. In spite of this, anti-obesity effect was not reached by two-week palm-LEAP2(1-14) treatment in mice with diet-induced obesity. Similarly, palm-LEAP2(1-14) administered simultaneously with high-fat diet feeding for 8 weeks did not protect mice from development of obesity and related biochemical changes. However, palm-LEAP2(1-14) kept its ability to attenuate liver de novo lipogenesis, and prominently lowered liver gene expression of nuclear receptors PPARG, SREBF1 and PPARA, and also expression of lipogenic and lipolytic enzymes. In our recent study, we described a high-fat diet-induced ghrelin resistance, reversible by switch to standard diet, followed by resistance to the acute anorexigenic effects of palm-LEAP2(1-14). Here we conclude that this resistance to palm-LEAP2(1-14) in obesity is probably selective and does not concern its ability to inhibit liver lipid metabolism.

The Coding Logic of Interoception

Interoception, the ability to precisely and timely sense internal body signals, is critical for life. The interoceptive system monitors a large variety of mechanical, chemical, hormonal, and pathological cues using specialized organ cells, organ innervating neurons, and brain sensory neurons. It is important for maintaining body homeostasis, providing motivational drives, and regulating autonomic, cognitive, and behavioral functions. However, compared to external sensory systems, our knowledge about how diverse body signals are coded at a system level is quite limited. In this review, we focus on the unique features of interoceptive signals and the organization of the interoceptive system, with the goal of better understanding the coding logic of interoception.

Ghrelin-responsive mediobasal hypothalamic neurons mediate exercise-associated food intake and exercise endurance

Previous studies have implicated the orexigenic hormone ghrelin as a mediator of exercise endurance and the feeding response postexercise. Specifically, plasma ghrelin levels nearly double in mice when they are subjected to an hour-long bout of high-intensity interval exercise (HIIE) using treadmills. Also, growth hormone secretagogue receptor-null (GHSR-null) mice exhibit decreased food intake following HIIE and diminished running distance (time until exhaustion) during a longer, stepwise exercise endurance protocol. To investigate whether ghrelin-responsive mediobasal hypothalamus (MBH) neurons mediate these effects, we stereotaxically delivered the inhibitory designer receptor exclusively activated by designer drugs virus AAV2-hSyn-DIO-hM4(Gi)-mCherry to the MBH of Ghsr-IRES-Cre mice, which express Cre recombinase directed by the Ghsr promoter. We found that chemogenetic inhibition of GHSR-expressing MBH neurons (upon delivery of clozapine-N-oxide) 1) suppressed food intake following HIIE, 2) reduced maximum running distance and raised blood glucose and blood lactate levels during an exercise endurance protocol, 3) reduced food intake following ghrelin administration, and 4) did not affect glucose tolerance. Further, HIIE increased MBH Ghsr expression. These results indicate that activation of ghrelin-responsive MBH neurons is required for the normal feeding response to HIIE and the usual amount of running exhibited during an exercise endurance protocol.

Ghrelin signalling in AgRP neurons links metabolic state to the sensory regulation of AgRP neural activity

OBJECTIVE

The sensory detection of food and food cues suppresses Agouti related peptide (AgRP) neuronal activity prior to consumption with greatest suppression occurring in response to highly caloric food or interoceptive energy need. However, the interoceptive mechanisms priming an appropriate AgRP neural response to external sensory information of food availability remain unexplored. Since hunger increases plasma ghrelin, we hypothesized that ghrelin receptor (GHSR) signalling on AgRP neurons is a key interoceptive mechanism integrating energy need with external sensory cues predicting caloric availability.

METHODS

We used in vivo photometry to measure the effects of ghrelin administration or fasting on AgRP neural activity with GCaMP6s and dopamine release in the nucleus accumbens with GRAB-DA in mice lacking ghrelin receptors in AgRP neurons.

RESULTS

The deletion of GHSR on AgRP neurons prevented ghrelin-induced food intake, motivation and AgRP activity. The presentation of food (peanut butter pellet) or a wooden dowel suppressed AgRP activity in fasted WT but not mice lacking GHSRs in AgRP neurons. Similarly, peanut butter and a wooden dowel increased dopamine release in the nucleus accumbens after ip ghrelin injection in WT but not mice lacking GHSRs in AgRP neurons. No difference in dopamine release was observed in fasted mice. Finally, ip ghrelin administration did not directly increase dopamine neural activity in the ventral tegmental area.

CONCLUSIONS

Our results suggest that AgRP GHSRs integrate an interoceptive state of energy need with external sensory information to produce an optimal change in AgRP neural activity. Thus, ghrelin signalling on AgRP neurons is more than just a feedback signal to increase AgRP activity during hunger.

mTORC1 Signaling in AgRP Neurons Is Not Required to Induce Major Neuroendocrine Adaptations to Food Restriction

Hypothalamic mTORC1 signaling is involved in nutrient sensing. Neurons that express the agouti-related protein (AgRP) are activated by food restriction and integrate interoceptive and exteroceptive signals to control food intake, energy expenditure, and other metabolic responses. To determine whether mTORC1 signaling in AgRP neurons is necessary for regulating energy and glucose homeostasis, especially in situations of negative energy balance, mice carrying ablation of the Raptor gene exclusively in AgRP-expressing cells were generated. AgRPΔRaptor mice showed no differences in body weight, fat mass, food intake, or energy expenditure; however, a slight improvement in glucose homeostasis was observed compared to the control group. When subjected to 5 days of food restriction (40% basal intake), AgRPΔRaptor female mice lost less lean body mass and showed a blunted reduction in energy expenditure, whereas AgRPΔRaptor male mice maintained a higher energy expenditure compared to control mice during the food restriction and 5 days of refeeding period. AgRPΔRaptor female mice did not exhibit the food restriction-induced increase in serum corticosterone levels. Finally, although hypothalamic fasting- or refeeding-induced Fos expression showed no differences between the groups, AgRPΔRaptor mice displayed increased hyperphagia during refeeding. Thus, some metabolic and neuroendocrine responses to food restriction are disturbed in AgRPΔRaptor mice.

CaMK1D signalling in AgRP neurons promotes ghrelin-mediated food intake

Hypothalamic AgRP/NPY neurons are key players in the control of feeding behaviour. Ghrelin, a major orexigenic hormone, activates AgRP/NPY neurons to stimulate food intake and adiposity. However, cell-autonomous ghrelin-dependent signalling mechanisms in AgRP/NPY neurons remain poorly defined. Here we show that calcium/calmodulin-dependent protein kinase ID (CaMK1D), a genetic hot spot in type 2 diabetes, is activated upon ghrelin stimulation and acts in AgRP/NPY neurons to mediate ghrelin-dependent food intake. Global Camk1d-knockout male mice are resistant to ghrelin, gain less body weight and are protected against high-fat-diet-induced obesity. Deletion of Camk1d in AgRP/NPY, but not in POMC, neurons is sufficient to recapitulate above phenotypes. In response to ghrelin, lack of CaMK1D attenuates phosphorylation of CREB and CREB-dependent expression of the orexigenic neuropeptides AgRP/NPY in fibre projections to the paraventricular nucleus (PVN). Hence, CaMK1D links ghrelin action to transcriptional control of orexigenic neuropeptide availability in AgRP neurons.

CPT1A in AgRP neurons is required for sex-dependent regulation of feeding and thirst

BACKGROUND

Fatty acid metabolism in the hypothalamus has an important role in food intake, but its specific role in AgRP neurons is poorly understood. Here, we examined whether carnitinea palmitoyltransferase 1A (CPT1A), a key enzyme in mitochondrial fatty acid oxidation, affects energy balance.

METHODS

To obtain Cpt1aKO mice and their control littermates, Cpt1a(flox/flox) mice were crossed with tamoxifen-inducible AgRPCreERT2 mice. Food intake and body weight were analyzed weekly in both males and females. At 12 weeks of age, metabolic flexibility was determined by ghrelin-induced food intake and fasting-refeeding satiety tests. Energy expenditure was analyzed by calorimetric system and thermogenic activity of brown adipose tissue. To study fluid balance the analysis of urine and water intake volumes; osmolality of urine and plasma; as well as serum levels of angiotensin and components of RAAS (renin-angiotensin-aldosterone system) were measured. At the central level, changes in AgRP neurons were determined by: (1) analyzing specific AgRP gene expression in RiboTag-Cpt1aKO mice obtained by crossing Cpt1aKO mice with RiboTag mice; (2) measuring presynaptic terminal formation in the AgRP neurons with the injection of the AAV1-EF1a-DIO-synaptophysin-GFP in the arcuate nucleus of the hypothalamus; (3) analyzing AgRP neuronal viability and spine formations by the injection AAV9-EF1a-DIO-mCherry in the arcuate nucleus of the hypothalamus; (4) analyzing in situ the specific AgRP mitochondria in the ZsGreen-Cpt1aKO obtained by breeding ZsGreen mice with Cpt1aKO mice. Two-way ANOVA analyses were performed to determine the contributions of the effect of lack of CPT1A in AgRP neurons in the sex.

RESULTS

Changes in food intake were just seen in male Cpt1aKO mice while only female Cpt1aKO mice increased energy expenditure. The lack of Cpt1a in the AgRP neurons enhanced brown adipose tissue activity, mainly in females, and induced a substantial reduction in fat deposits and body weight. Strikingly, both male and female Cpt1aKO mice showed polydipsia and polyuria, with more reduced serum vasopressin levels in females and without osmolality alterations, indicating a direct involvement of Cpt1a in AgRP neurons in fluid balance. AgRP neurons from Cpt1aKO mice showed a sex-dependent gene expression pattern, reduced mitochondria and decreased presynaptic innervation to the paraventricular nucleus, without neuronal viability alterations.

CONCLUSIONS

Our results highlight that fatty acid metabolism and CPT1A in AgRP neurons show marked sex differences and play a relevant role in the neuronal processes necessary for the maintenance of whole-body fluid and energy balance.

The expression and function of growth hormone secretagogue receptor in immune cells: A current perspective

The orexigenic hormone ghrelin and its receptor, growth hormone secretagogue receptor (GHS-R), have been extensively studied in the last two decades, revealing that ghrelin signaling has important implications in health and disease. Metabolic diseases, such as obesity and diabetes, are often accompanied by low-grade chronic inflammation, that has been coined as "meta-inflammation." Immune cells are key cellular mediators of meta-inflammation, controlling both initiation and resolution of inflammation. Immune cells exhibit dynamic changes in cellular characteristics and functional output in response to the stimuli/insults from their surrounding microenvironment. Emerging evidence shows that ghrelin has an important effect on inflammation, in addition to its well-known effects on metabolism. However, the cellular/molecular mechanism of ghrelin signaling in immunity is largely unknown because the knowledge in regard to the expression and function of GHS-R in immune cells is currently sparse. In this review, we have accumulated the recent findings related to the expression and functions of GHS-R in various immune cells under different physiological and pathological states. This review aims to inspire further investigation of the immunological roles of ghrelin signaling and advance the therapeutic applications of ghrelin signaling in meta-inflammation.

Deletion of ghrelin alters tryptophan metabolism and exacerbates experimental ulcerative colitis in aged mice

A major component of aging is chronic, low-grade inflammation, attributable in part by impaired gut barrier function. We previously reported that deletion of ghrelin, a peptidergic hormone released mainly from the gut, exacerbates experimental muscle atrophy in aged mice. In addition, ghrelin has been shown to ameliorate colitis in experimental models of inflammatory bowel disease (IBD), although the role of endogenous ghrelin in host-microbe interactions is less clear. Here, we showed that 22-month-old global ghrelin knockout (Ghrl^-/-) mice exhibited significantly increased depressive-like behaviors, while anxiety levels and working memory were similar to littermate wild-type (WT) mice. Furthermore, old Ghrl^-/- mice showed significantly increased intestinal permeability to fluorescein isothiocyanate (FITC)-dextran, significantly higher colonic interleukin (IL-1β) levels, and trends for higher colonic IL-6 and tumor necrosis factor-α (TNF-α) compared to WT mice. Interestingly, young Ghrl^-/- and WT mice showed comparable depressive-like behavior and gut permeability, suggesting age-dependent exacerbation in gut barrier dysfunction in Ghrl^-/- mice. While fecal short-chain fatty acids levels were comparable between old Ghrl^-/- and WT mice, serum metabolome revealed alterations in metabolic cascades including tryptophan metabolism. Specifically, tryptophan and its microbial derivatives indole-3-acetic acid and indole-3-lactic acid were significantly reduced in old Ghrl^-/-mice. Furthermore, in an experimental model of dextran sulfate sodium (DSS)-induced colitis, Ghrl^-/- mice showed exacerbated disease symptoms, and higher levels of chemoattractant and pro-inflammatory cytokines in the colon. Overall, these data demonstrated that ghrelin deficiency is associated with gut barrier dysfunction, alterations in microbially derived tryptophan metabolites, and increased susceptibility to colitis. These data suggested that endogenous ghrelin contributes to maintaining a healthy host-microbe environment, ultimately impacting on brain function.

Characterization of the metabolic differences between male and female C57BL/6 mice

AIMS

The present study aims to compare the responses between male and female C57BL/6 mice to multiple metabolic challenges to understand the importance of sex in the control of energy homeostasis.

MAIN METHODS

Male and female C57BL/6 mice were subjected to nutritional and hormonal challenges, such as food restriction and refeeding, diet-induced obesity, feeding response to ghrelin and leptin, ghrelin-induced growth hormone secretion, and central responsiveness to ghrelin and leptin. The hypothalamic expression of transcripts that control energy homeostasis was also evaluated.

KEY FINDINGS

Male mice lost more weight and lean body mass in response to food restriction, compared to females. During refeeding, males accumulated more body fat and exhibited lower energy expenditure and glycemia, as compared to females. Additionally, female mice exhibited a higher protection against diet-induced obesity and related metabolic imbalances in comparison to males. Low dose ghrelin injection elicited higher food intake and growth hormone secretion in male mice, whereas the acute anorexigenic effect of leptin was more robust in females. However, the sex differences in the feeding responses to ghrelin and leptin were not explained by variations in the central responsiveness to these hormones nor by differences in the fiber density from arcuate nucleus neurons. Female, but not male, mice exhibited compensatory increases in hypothalamic Pomc mRNA levels in response to diet-induced obesity.

SIGNIFICANCE

Our findings revealed several sexually differentiated responses to metabolic challenges in C57BL/6 mice, highlighting the importance of taking into account sex differences in metabolic studies.

Arcuate Nucleus-Dependent Regulation of Metabolism-Pathways to Obesity and Diabetes Mellitus

The central nervous system (CNS) receives information from afferent neurons, circulating hormones, and absorbed nutrients and integrates this information to orchestrate the actions of the neuroendocrine and autonomic nervous systems in maintaining systemic metabolic homeostasis. Particularly the arcuate nucleus of the hypothalamus (ARC) is of pivotal importance for primary sensing of adiposity signals, such as leptin and insulin, and circulating nutrients, such as glucose. Importantly, energy state-sensing neurons in the ARC not only regulate feeding but at the same time control multiple physiological functions, such as glucose homeostasis, blood pressure, and innate immune responses. These findings have defined them as master regulators, which adapt integrative physiology to the energy state of the organism. The disruption of this fine-tuned control leads to an imbalance between energy intake and expenditure as well as deregulation of peripheral metabolism. Improving our understanding of the cellular, molecular, and functional basis of this regulatory principle in the CNS could set the stage for developing novel therapeutic strategies for the treatment of obesity and metabolic syndrome. In this review, we summarize novel insights with a particular emphasis on ARC neurocircuitries regulating food intake and glucose homeostasis and sensing factors that inform the brain of the organismal energy status.

"Sibling" battle or harmony: crosstalk between nesfatin-1 and ghrelin

Ghrelin was first identified as an endogenous ligand of the growth hormone secretagogue receptor (GHSR) in 1999, with the function of stimulating the release of growth hormone (GH), while nesfatin-1 was identified in 2006. Both peptides are secreted by the same kind of endocrine cells, X/A-like cells in the stomach. Compared with ghrelin, nesfatin-1 exerts opposite effects on energy metabolism, glucose metabolism, gastrointestinal functions and regulation of blood pressure, but exerts similar effects on anti-inflammation and neuroprotection. Up to now, nesfatin-1 remains as an orphan ligand because its receptor has not been identified. Several studies have shown the effects of nesfatin-1 are dependent on the receptor of ghrelin. We herein compare the effects of nesfatin-1 and ghrelin in several aspects and explore the possibility of their interactions.

Neuronal GHS-R Differentially Modulates Feeding Patterns under Normal and Obesogenic Conditions

The orexigenic hormone ghrelin increases food intake and promotes obesity through its receptor, growth hormone secretagogue receptor (GHS-R). We previously reported two neuron-specific GHS-R knockout mouse lines, namely pan-neuronal deletion by Syn1-cre and hypothalamic deletion by AgRP-cre, exhibiting differential diet-dependent effects on body weight. GHS-R deficiency in neurons elicited less pronounced metabolic effects under regular diet (RD) than high fat diet (HFD). While there was no difference in total food intake of HFD in either mouse line, Syn1-cre; Ghsrf/f mice showed much greater anti-obesity effect than that of AgRP-cre; Ghsrf/f mice. Meal feeding pattern is known to have a major impact on energy homeostasis and obesity development. Here, we investigated the feeding behaviors of these two neuron-specific GHS-R knockout mice under RD and HFD feeding, by assessing meal number, meal size, meal duration, and feeding frequency. Under the normal diet, RD-fed Syn1-cre; Ghsrf/f mice showed a decreased meal size in dark phase, while RD-fed AgRP-cre; Ghsrf/f mice showed an increased meal duration in dark phase. Under the obesogenic diet, HFD-fed Syn1-cre; Ghsrf/f mice displayed reduced meal numbers in light phase and increased feeding in both light and dark phases, whereas HFD-fed AgRP-cre; Ghsrf/f mice showed a decreased meal duration in the light phase only. Consistently, the expression of neuropeptides (Neuropeptide Y and Orexin) was increased in the hypothalamus of RD-fed Syn1-cre; Ghsrf/f mice, whereas the expression of cannabinoid receptor type 1 (CB1) was increased in the hypothalamus of HFD fed Syn1-cre; Ghsrf/f mice. Overall, feeding pattern changes were more pronounced in Syn1-cre; Ghsrf/f mice than that in AgRP-cre; Ghsrf/f mice, and HFD elicited greater alteration than RD. While AgRP-cre; Ghsrf/f mice consumed HFD meals faster during the day (showing shorter meal duration), Syn1-cre; Ghsrf/f mice ate few HFD meals during the light phase and ate slowly throughout the day (showing longer meal duration in both phases). Our findings reveal that neuronal GHS-R regulates energy homeostasis by altering feeding patterns, and differentially modulates feeding patterns in a site- and diet-dependent manner. The distinctive data in these two mouse lines also suggest that eating slowly during the optimal feeding period (dark phase for mice) may be beneficial in combating obesity.

AgRP/NPY and POMC neurons in the arcuate nucleus and their potential role in treatment of obesity

Obesity is a major health crisis affecting over a third of the global population. This multifactorial disease is regulated via interoceptive neural circuits in the brain, whose alteration results in excessive body weight. Certain central neuronal populations in the brain are recognised as crucial nodes in energy homeostasis; in particular, the hypothalamic arcuate nucleus (ARC) region contains two peptide microcircuits that control energy balance with antagonistic functions: agouti-related peptide/neuropeptide-Y (AgRP/NPY) signals hunger and stimulates food intake; and pro-opiomelanocortin (POMC) signals satiety and reduces food intake. These neuronal peptides levels react to energy status and integrate signals from peripheral ghrelin, leptin, and insulin to regulate feeding and energy expenditure. To manage obesity comprehensively, it is crucial to understand cellular and molecular mechanisms of information processing in ARC neurons, since these regulate energy homeostasis. Importantly, a specific strategy focusing on ARC circuits needs to be devised to assist in treating obese patients and maintaining weight loss with minimal or no side effects. The aim of this review is to elucidate the recent developments in the study of AgRP-, NPY- and POMC-producing neurons, specific to their role in controlling metabolism. The impact of ghrelin, leptin, and insulin signalling via action of these neurons is also surveyed, since they also impact energy balance through this route. Lastly, we present key proteins, targeted genes, compounds, drugs, and therapies that actively work via these neurons and could potentially be used as therapeutic targets for treating obesity conditions.

Los posibles mecanismos de pardeamiento del tejido adiposo blanco: una diana novedosa para el tratamiento de la obesidad

The increase of the obesity pandemic worldwide over the last several decades has generated a constant need for the scientific world to develop new possibilities to combat obesity. Since the discovery that brown adipose tissue (BAT) exists in adult humans, and BAT activation contributes to a negative energy balance, much more attention has been focused on the understanding of the molecular switches and their different regulatory mechanisms turning on energy expenditure. Recent insights have revealed that a range of stimuli including cold exposure, physical activity and diet, and critical transcription molecules such as PPARγ, PRDM16, PGC-1α and UCP1, aiming at the induction of BAT activation, could cause the browning of white adipose tissue, thereby dissipating energy and increasing heat production. An increasing number of studies that point to the white adipose tissue (WAT) browning strategies aiming at diet-induced and/or genetically determined obesity have been tested in mouse models as well as in human studies. Findings suggested that browning stimulating drugs have been currently or previously assayed as a therapy against obesity. As PPARα agonists, fibrate drugs effectively reduced plasma triglyceride, increased high-density lipoproteins, and improved glycemic control and heat production in brown adipose tissue, which has been used in the treatment of metabolic disorders. Many kinds of natural products promote white adipose tissue browning, such as alkaloids, flavonoids, terpenoids, and long-chain fatty acids, which can also ameliorate metabolic disorders including obesity, insulin resistance and diabetes. The aim of this review is to summarize the transcriptional regulators as well as the various mediators that have been regarded as potential therapeutic targets in the process of WAT browning.

A skeleton in the cupboard in ghrelin research: Where are the skinny dwarfs?

Based on studies delivering ghrelin or ghrelin receptor agonists, we have learned a great deal about the importance of the brain ghrelin signalling system for a wide range of physiological processes that include feeding behaviours, growth hormone secretion and glucose homeostasis. Because these processes can be considered as essential to life, the question arises as to why mouse models of depleted ghrelin signalling are not all skinny dwarfs with a host of behavioural and metabolic problems. Here, we provide a systematic detailed review of the phenotype of mice with deficient ghrelin signalling to help better understand the relevance and importance of the brain ghrelin signalling system, with a particular emphasis on those questions that remain unanswered.

Ghrelin Alleviates Endoplasmic Reticulum Stress in MC3T3E1 Cells by Inhibiting AMPK Phosphorylation

Ghrelin is a gastric endocrine peptide that has been found to be involved in the process of energy homeostasis and bone physiology in recent years. To explore the effects of ghrelin on endoplasmic reticulum stress (ERS) in MC3T3E1 cells and its possible mechanism, an ERS model was induced by tunicamycin (TM) in the osteoblast line MC3T3E1. TM at 1.5 μg/mL was selected as the experimental concentration found by CCK8 assay. Through the determination of apoptosis, reactive oxygen species production, and endoplasmic reticulum stress-related gene expression, we found that ERS induced by TM can be relieved by ghrelin in a concentration-dependent manner (P < 0.001). Compared with the TM group, ghrelin reduced the expression of ERS-related marker genes induced by TM. Compared with the GSK621 + TM group without ghrelin pretreatment, the mRNA expression of genes in the ghrelin pretreatment group decreased significantly (P < 0.001). The results of protein analysis showed that the levels of BIP, p-AMPK, and cleaved-caspase3 in the TM group increased significantly, while the levels decreased after ghrelin pretreatment. In group GSK621 + TM compared with group GSK621 + ghrelin+TM, ghrelin pretreatment significantly reduced the level of p-AMPK, which is consistent with the trend of the ERS-related proteins BIP and cleaved-caspase3. In conclusion, ghrelin alleviates the ERS induced by TM in a concentration-dependent manner and may or at least partly alleviate the apoptosis induced by ERS in MC3T3E1 cells by inhibiting the phosphorylation of AMPK.

Ghrelin treatment induces rapid and delayed increments of food intake: a heuristic model to explain ghrelin's orexigenic effects

Ghrelin is a stomach-derived peptide hormone with salient roles in the regulation of energy balance and metabolism. Notably, ghrelin is recognized as the most powerful known circulating orexigenic hormone. Here, we systematically investigated the effects of ghrelin on energy homeostasis and found that ghrelin primarily induces a biphasic effect on food intake that has indirect consequences on energy expenditure and nutrient partitioning. We also found that ghrelin-induced biphasic effect on food intake requires the integrity of Agouti-related peptide/neuropeptide Y-producing neurons of the hypothalamic arcuate nucleus, which seem to display a long-lasting activation after a single systemic injection of ghrelin. Finally, we found that different autonomic, hormonal and metabolic satiation signals transiently counteract ghrelin-induced food intake. Based on our observations, we propose a heuristic model to describe how the orexigenic effect of ghrelin and the anorectic food intake-induced rebound sculpt a timely constrain feeding response to ghrelin.

Effects of Growth Hormone Receptor Ablation in Corticotropin-Releasing Hormone Cells

Corticotropin-releasing hormone (CRH) cells are the dominant neuronal population responsive to the growth hormone (GH) in the paraventricular nucleus of the hypothalamus (PVH). However, the physiological importance of GH receptor (GHR) signaling in CRH neurons is currently unknown. Thus, the main objective of the present study was to investigate the consequences of GHR ablation in CRH-expressing cells of male and female mice. GHR ablation in CRH cells did not cause significant changes in body weight, body composition, food intake, substrate oxidation, locomotor activity, glucose tolerance, insulin sensitivity, counterregulatory response to 2-deoxy-D-glucose and ghrelin-induced food intake. However, reduced energy expenditure was observed in female mice carrying GHR ablation in CRH cells. The absence of GHR in CRH cells did not affect anxiety, circadian glucocorticoid levels or restraint-stress-induced corticosterone secretion and activation of PVH neurons in both male and female mice. In summary, GHR ablation, specifically in CRH-expressing neurons, does not lead to major alterations in metabolism, hypothalamic-pituitary-adrenal axis, acute stress response or anxiety in mice. Considering the previous studies showing that central GHR signaling regulates homeostasis in situations of metabolic stress, future studies are still necessary to identify the potential physiological importance of GH action on CRH neurons.

GHS-R suppression in adipose tissues protects against obesity and insulin resistance by regulating adipose angiogenesis and fibrosis

BACKGROUND/OBJECTIVES

Ghrelin is an orexigenic hormone that increases food intake, adiposity, and insulin resistance through its receptor Growth Hormone Secretagogue Receptor (GHS-R). We previously showed that ghrelin/GHS-R signaling has important roles in regulation of energy homeostasis, and global deletion of GHS-R reduces obesity and improves insulin sensitivity by increasing thermogenesis. However, it is unknown whether GHS-R regulates thermogenic activation in adipose tissues directly.

METHODS

We generated a novel adipose tissue-specific GHS-R deletion mouse model and characterized the mice under regular diet (RD) and high-fat diet (HFD) feeding. Body composition was measured by Echo MRI. Metabolic profiling was determined by indirect calorimetry. Response to environmental stress was assessed using a TH-8 temperature monitoring system. Insulin sensitivity was evaluated by glucose and insulin tolerance tests. Tissue histology was analyzed by hematoxylin/eosin and immunofluorescent staining. Expression of genes involved in thermogenesis, angiogenesis and fibrosis in adipose tissues were analyzed by real-time PCR.

RESULTS

Under RD feeding, adipose tissue-specific GHS-R deletion had little or no impact on metabolic parameters. However, under HFD feeding, adipose tissue-specific GHS-R deletion attenuated diet-induced obesity and insulin resistance, showing elevated physical activity and heat production. In addition, adipose tissue-specific GHS-R deletion increased expression of master adipose transcription regulator of peroxisome proliferator-activated receptor (PPAR) γ1 and adipokines of adiponectin and fibroblast growth factor (FGF) 21; and differentially modulated angiogenesis and fibrosis evident in both gene expression and histological analysis.

CONCLUSIONS

These results show that GHS-R has cell-autonomous effects in adipocytes, and suppression of GHS-R in adipose tissues protects against diet-induced obesity and insulin resistance by modulating adipose angiogenesis and fibrosis. These findings suggest adipose GHS-R may constitute a novel therapeutic target for treatment of obesity and metabolic syndrome.

Ghrelin-induced Food Intake, but not GH Secretion, Requires the Expression of the GH Receptor in the Brain of Male Mice

Ghrelin stimulates both GH secretion and food intake. The orexigenic action of ghrelin is mainly mediated by neurons that coexpress agouti-related protein (AgRP) and neuropeptide Y (NPY) in the arcuate nucleus of the hypothalamus (ARH). GH also stimulates food intake and, importantly, ARHAgRP/NPY neurons express GH receptor (GHR). Thus, ghrelin-induced GH secretion may contribute to the orexigenic effect of ghrelin. Here, we investigated the response to ghrelin in male mice carrying GHR ablation specifically in neurons (brain GHR knockout [KO] mice) or exclusively in ARHAgRP/NPY neurons (AgRP GHR KO mice). Although brain GHR KO mice showed normal ghrelin-induced increase in plasma GH levels, these mutants lacked the expected orexigenic response to ghrelin. Additionally, brain GHR KO mice displayed reduced hypothalamic levels of Npy and Ghsr mRNA and did not elicit ghrelin-induced c-Fos expression in the ARH. Furthermore, brain GHR KO mice exhibited a prominent reduction in AgRP fiber density in the ARH and paraventricular nucleus of the hypothalamus (PVH). In contrast, AgRP GHR KO mice showed no changes in the hypothalamic Npy and Ghsr mRNAs and conserved ghrelin-induced food intake and c-Fos expression in the ARH. AgRP GHR KO mice displayed a reduced AgRP fiber density (~16%) in the PVH, but this reduction was less than that observed in brain GHR KO mice (~61%). Our findings indicate that GHR signaling in the brain is required for the orexigenic effect of ghrelin, independently of GH action on ARHAgRP/NPY neurons.

Disrupting the ghrelin-growth hormone axis limits ghrelin's orexigenic but not glucoregulatory actions

Objective

Acyl-ghrelin regulates eating, body weight, blood glucose, and GH secretion upon binding to its receptor GHSR (growth hormone secretagogue receptor; ghrelin receptor). GHSR is distributed in several brain regions and some peripheral cell-types including pituitary somatotrophs. The objective of the current study was to determine the functional significance of acyl-ghrelin's action on GHSR-expressing somatotrophs in mediating GH secretion and several of acyl-ghrelin's metabolic actions.

Methods

GH-IRES-Cre mice and loxP-flanked (floxed) GHSR mice were newly developed and then crossed to one another to generate mice that lacked GHSR selectively from somatotrophs. Following validation of mice with somatotroph-selective GHSR deletion, metabolic responses of these mice and control littermates were assessed following both acute and chronic acyl-ghrelin administration, a 24-h fast, and a prolonged 60% chronic caloric restriction protocol modeling starvation.

Results

In mice with somatotroph-selective GHSR deletion, a single peripheral injection of acyl-ghrelin failed to induce GH secretion or increase food intake, unlike wild-type and other littermate control groups. However, the usual acute blood glucose increase in response to the acyl-ghrelin bolus was preserved. Similarly, chronic s.c. acyl-ghrelin administration to mice with somatotroph-selective GHSR deletion failed to increase plasma GH, food intake, or body weight. Physiologically elevating plasma acyl-ghrelin via a 24-h fast also failed to raise plasma GH and resulted in a limited hyperphagic response upon food reintroduction in mice with somatotroph-selective GHSR deletion, although those mice nonetheless did not exhibit an exaggerated reduction in blood glucose. Physiologically elevating plasma acyl-ghrelin via a 15-day caloric restriction protocol which provided only 40% of usual daily calories failed to raise plasma GH in mice with somatotroph-selective GHSR deletion, although those mice did not exhibit life-threatening hypoglycemia.

Conclusions

These results reveal that direct engagement of GHSR-expressing somatotrophs is required for a peripheral ghrelin bolus to acutely stimulate GH secretion and the actions of chronic acyl-ghrelin delivery and physiological plasma acyl-ghrelin elevations to increase plasma GH. These results also suggest that actions of acyl-ghrelin to increase food intake and body weight are reliant on direct activation of GHSRs expressed on somatotrophs. Furthermore, these results suggest that the glucoregulatory actions of acyl-ghrelin – in particular, its actions to raise blood glucose when acutely administered, prevent small blood glucose drops following a 24-h fast, and avert life-threatening hypoglycemia during an acute-on-chronic caloric restriction protocol – do not depend on GHSR expression by somatotrophs.

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Mice with pituitary somatotroph-selective GHSR deletion were generated.

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Somatotroph-expressed GHSRs mediate GH secretion and food intake after acute ghrelin.

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Body weight effects of chronic ghrelin infusion require somatotroph-expressed GHSRs.

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Somatotroph-expressed GHSRs enable GH to increase upon chronic caloric restriction.

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Mice lacking somatotroph GHSRs maintain euglycemia upon chronic caloric restriction.

GHS-R in brown fat potentiates differential thermogenic responses under metabolic and thermal stresses

In response to cold or diet, fatty acids are dissipated into heat through uncoupling protein 1 (UCP1) in brown adipose tissue (BAT). This process is termed non-shivering thermogenesis, which is important for body temperature maintenance and contributes to obesity pathogenesis. Thermogenic enhancement has been considered a promising anti-obesity strategy. Ghrelin and its receptor Growth Hormone Secretagogue Receptor (GHS-R) have critical roles in energy intake, nutrient sensing, and lipid metabolism. We previously reported that global Ghsr-knockout mice have increased energy expenditure due to enhanced thermogenesis. To determine the site of action for GHS-R mediated thermogenesis, we generated brown adipocyte-specific Ghsr knockout mice (UCP1-CreER/Ghsrf/f) and assessed thermogenic responses under regular diet (RD) fed homeostatic metabolic state or high-fat diet (HFD) fed metabolically-impaired obese state, under normal or cold housing environment. Under a RD-feeding, UCP1-CreER/Ghsrf/f mice showed increased body fat and a slightly elevated core body temperature under cold but not under normal temperature. Consistently, the expression of thermogenic genes in BAT of RD-fed UCP1-CreER/Ghsrf/f mice was increased in reposes to cold. Under HFD feeding, HFD-fed UCP1-CreER/Ghsrf/f mice showed no difference in body fat or body temperature under either normal or cold exposure. Interestingly, the expression of thermogenic genes in BAT of HFD-fed UCP1-CreER/Ghsrf/f mice was upregulated under normal temperature but downregulated under cold exposure. Overall, our data show that GHS-R has cell-autonomous effect in brown adipocytes, and GHS-R regulates BAT thermogenic activity in a temperature- and metabolic state-dependent manner. The thermogenic effect of GHS-R in BAT is more pronounced in cold environment and differentially variable based on metabolic state; under cold exposure, GHS-R inhibition in BAT activates thermogenesis under homeostatic state but suppresses thermogenesis under obese state. Our finding collectively suggests that GHS-R in BAT, acting as a "metabolic thermostat", differentially regulates thermogenesis in response to different metabolic and thermal stimuli.

Functional and Neurochemical Identification of Ghrelin Receptor (GHSR)-Expressing Cells of the Lateral Parabrachial Nucleus in Mice

The lateral parabrachial nucleus (lPBN), located in the pons, is a well-recognized anorexigenic center harboring, amongst others, the calcitonin gene-related peptide (CGRP)-expressing neurons that play a key role. The receptor for the orexigenic hormone ghrelin (the growth hormone secretagogue receptor, GHSR) is also abundantly expressed in the lPBN and ghrelin delivery to this site has recently been shown to increase food intake and alter food choice. Here we sought to explore whether GHSR-expressing cells in the lPBN (GHSR ^lPBN cells) contribute to feeding control, food choice and body weight gain in mice offered an obesogenic diet, involving studies in which GHSR ^lPBN cells were silenced. We also explored the neurochemical identity of GHSR ^lPBN cells. To silence GHSR ^lPBN cells, Ghsr-IRES-Cre male mice were bilaterally injected intra-lPBN with a Cre-dependent viral vector expressing tetanus toxin-light chain. Unlike control wild-type littermates that significantly increased in body weight on the obesogenic diet (i.e., high-fat high-sugar free choice diet comprising chow, lard and 9% sucrose solution), the heterozygous mice with silenced GHSR ^lPBN cells were resistant to diet-induced weight gain with significantly lower food intake and fat weight. The lean phenotype appeared to result from a decreased food intake compared to controls and caloric efficiency was unaltered. Additionally, silencing the GHSR ^lPBN cells altered food choice, significantly reducing palatable food consumption. RNAscope and immunohistochemical studies of the lPBN revealed considerable co-expression of GHSR with glutamate and pituitary adenylate cyclase-activating peptide (PACAP), and much less with neurotensin, substance P and CGRP. Thus, the GHSR ^lPBN cells are important for diet-induced weight gain and adiposity, as well as in the regulation of food intake and food choice. Most GHSR ^lPBN cells were found to be glutamatergic and the majority (76%) do not belong to the well-characterized anorexigenic CGRP cell population.

Metabolic regulation of kisspeptin - the link between energy balance and reproduction

Hypothalamic kisspeptin neurons serve as the nodal regulatory centre of reproductive function. These neurons are subjected to a plethora of regulatory factors that ultimately affect the release of kisspeptin, which modulates gonadotropin-releasing hormone (GnRH) release from GnRH neurons to control the reproductive axis. The presence of sufficient energy reserves is critical to achieve successful reproduction. Consequently, metabolic factors impose a very tight control over kisspeptin synthesis and release. This Review offers a synoptic overview of the different steps in which kisspeptin neurons are subjected to metabolic regulation, from early developmental stages to adulthood. We cover an ample array of known mechanisms that underlie the metabolic regulation of KISS1 expression and kisspeptin release. Furthermore, the novel role of kisspeptin neurons as active players within the neuronal circuits that govern energy balance is discussed, offering evidence of a bidirectional role of these neurons as a nexus between metabolism and reproduction.

Ghrelin-insulin-like growth factor-1 axis is activated via autonomic neural circuits in the non-alcoholic fatty liver disease

BACKGROUND

The correlation of the growth hormone (GH) and insulin-like growth factor-1 (IGF-1) with non-alcoholic fatty liver disease (NAFLD) has been reported in epidemiological studies. However, the mechanisms of molecular and inter-organ systems that render these factors to influence on NAFLD have not been elucidated. In this study, we examined the induction of ghrelin which is the GH-releasing hormone and IGF-1, and involvement of autonomic neural circuits, in the pathogenesis of NAFLD.

METHODS

The expression of gastric and hypothalamic ghrelin, neural activation in the brain, and serum IGF-1 were examined in NAFLD models of choline-deficient defined l-amino-acid diet-fed, melanocortin 4 receptor knockout mice, and partial hepatectomy mice with or without the blockades of autonomic nerves to test the contribution of neural circuits connecting the brain, liver, and stomach.

KEY RESULTS

The fatty changes in the liver increased the expression of gastric ghrelin through the autonomic pathways which sends the neural signals to the arcuate nucleus in the hypothalamus through the afferent vagal nerve which reached the pituitary gland to release GH and then stimulate the IGF-1 release from the liver. In addition, high levels of ghrelin expression in the arcuate nucleus were correlated with NAFLD progression regardless of the circuits.

CONCLUSIONS

Our study demonstrated that the fatty liver stimulates the autonomic nervous signal circuits which suppress the progression of the disease by activating the gastric ghrelin expression, the neural signal transduction in the brain, and the release of IGF-1 from the liver.

The Ghrelin-AgRP Neuron Nexus in Anorexia Nervosa: Implications for Metabolic and Behavioral Adaptations

Anorexia Nervosa (AN) is viewed as primarily a psychiatric disorder owing to the considerable behavioral and genetic overlap with mood disorders and other psychiatric traits. However, the recent reconceptualization of AN as one of both psychiatric and metabolic etiology suggests that metabolic circuits conveying hunger, or sensitive to signals of hunger, may be a critical nexus linking metabolic dysfunction to mood disturbances. Within the brain, hunger is primarily percieved by Agouti-related (AgRP) neurons and hunger increases plasma concentrations of the hormone ghrelin, which targets ghrelin receptors on AgRP neurons to facilitate metabolic adaptations to low energy availability. However, beyond the fundamental role in maintaining hunger signaling, AgRP neurons regulate a diverse range of behaviors such as motivation, locomotor activity, negative reinforcement, anxiety, and obsession and a key factor involved in the manifestation of these behavioral changes in response to activation is the presence or absence of food availability. These changes can be considered adaptive in that they promote affective food-seeking strategies in environments with limited food availability. However, it also suggests that these neurons, so well-studied for their metabolic control, shape mood-related behaviors in a context-dependent manner and dysfunctional control leads not only to metabolic problems but also potentially mood-related problems. The purpose of this review is to underline the potential role of AgRP neurons and ghrelin signaling in both the metabolic and behavioral changes observed in anorexia nervosa. We aim to highlight the most recent studies on AgRP neurons and ghrelin signaling and integrate their metabolic and behavioral roles in normal function and highlight how dysfunction may contribute to the development of AN.

Ghrelin Signaling in Immunometabolism and Inflamm-Aging

Intracellular changes in immune cells lead to metabolic dysfunction, which is termed immunometabolism. Chronic inflammation is a hallmark of aging; this phenomenon is described as inflamm-aging. Immunometabolism and inflamm-aging are closely linked to obesity, insulin resistance, type 2 diabetes (T2D), cardiovascular diseases, and cancers, which consequently reduce life span and health span of the elderly. Ghrelin is an orexigenic hormone that regulates appetite and food intake. Ghrelin's functions are mediated through its receptor, growth hormone secretagogue receptor (GHS-R). Ghrelin and GHS-R have important roles in age-associated obesity, insulin resistance, and T2D. In this chapter, we have discussed the roles of ghrelin signaling in diet-induced obesity and normal aging as it relates to energy metabolism and inflammation in key metabolic tissues and organs. The new findings reveal that ghrelin signaling is an important regulatory mechanism for immunometabolism and inflamm-aging. Ghrelin signaling offers an exciting novel therapeutic strategy for treatment of obesity and insulin resistance of the elderly.

Ghrelin receptor in agouti-related peptide neurones regulates metabolic adaptation to calorie restriction

Ghrelin is a gut hormone that signals to the hypothalamus to stimulate growth hormone release, increase food intake and promote fat deposition. The ghrelin receptor, also known as growth hormone secretagogue receptor (GHS-R), is highly expressed in the brain, with the highest expression in agouti-related peptide (AgRP) neurones in the hypothalamus. Compelling evidence indicates that ghrelin serves as a survival hormone with respect to maintaining blood glucose and body weight during nutritional deficiencies. Recent studies have demonstrated that AgRP neurones are involved in metabolic and behavioural adaptation to an energy deficit to improve survival. In the present study, we used a neuronal subtype-specific GHS-R knockout mouse (AgRP-Cre;Ghsr^f/f ) to investigate the role of GHS-R in hypothalamic AgRP neurones in metabolic and behavioural adaptation to hypocaloric restricted feeding. We subjected the mice to a restricted feeding regimen of 40% mild calorie restriction (CR), with one-quarter of food allotment given in the beginning of the light cycle and three-quarters given at the beginning of the dark cycle, to mimic normal mouse intake pattern. The CR-fed AgRP-Cre;Ghsr^f/f mice exhibited reductions in body weight, fat mass and blood glucose. Metabolic profiling of these CR-fed AgRP-Cre;Ghsr^f/f mice showed a trend toward reduced basal metabolic rate, significantly reduced core body temperature and a decreased expression of thermogenic genes in brown adipose tissue. This suggests a metabolic reset to a lower threshold. Significantly increased physical activity, a trend toward increased food anticipatory behaviour and altered fuel preferences were also observed in these mice. In addition, these CR-fed AgRP-Cre;Ghsr^f/f mice exhibited a decreased counter-regulatory response, showing impaired hepatic glucose production. Lastly, hypothalamic gene expression in AgRP-Cre;Ghsr^f/f mice revealed increased AgRP expression and a decreased expression of genes in β-oxidation pathways. In summary, our data suggest that GHS-R in AgRP neurones is a key component of the neurocircuitry involved in metabolic adaptation to calorie restriction.

Adipose tissue browning in mice and humans

In the midst of an obesity epidemic, the promotion of brown adipose tissue (BAT) function and the browning of white adipose tissue (WAT) have emerged as promising therapeutic targets to increase energy expenditure and counteract weight gain. Despite the fact that the thermogenic potential of bone fide BAT in rodents is several orders of magnitudes higher than white fat containing brite/beige adipocytes, WAT browning represents a particularly intriguing concept in humans given the extreme amount of excess WAT in obese individuals. In addition, the clear distinction between classic brown and beige fat that has been proposed in mice does not exist in humans. In fact, studies of human BAT biopsies found controversial results suggesting both classic brown and beige characteristics. Irrespective of the true ‘color’, accumulating evidence suggests the induction of thermogenic adipocytes in human WAT depots in response to specific stimuli, highlighting that WAT browning may occur in both, mice and humans. These observations also emphasize the great plasticity of human fat depots and raise important questions about the metabolic properties of thermogenically active adipose tissue in humans and the potential therapeutic implications. We will first review the cellular and molecular aspects of selected adipose tissue browning concepts that have been identified in mouse models with emphasis on neuronal factors, the microbiome, immune cells and several hormones. We will also summarize the evidence for adipose tissue browning in humans including some experimental pharmacologic approaches.

Ghrelin's Relationship to Blood Glucose

Much effort has been directed at studying the orexigenic actions of administered ghrelin and the potential effects of the endogenous ghrelin system on food intake, food reward, body weight, adiposity, and energy expenditure. Although endogenous ghrelin's actions on some of these processes remain ambiguous, its glucoregulatory actions have emerged as well-recognized features during extreme metabolic conditions. The blood glucose-raising actions of ghrelin are beneficial during starvation-like conditions, defending against life-threatening falls in blood glucose, but they are seemingly detrimental in obese states and in certain monogenic forms of diabetes, contributing to hyperglycemia. Also of interest, blood glucose negatively regulates ghrelin secretion. This article reviews the literature suggesting the existence of a blood glucose-ghrelin axis and highlights the factors that mediate the glucoregulatory actions of ghrelin, especially during metabolic extremes such as starvation and diabetes.

aP2-Cre Mediated Ablation of GHS-R Attenuates Adiposity and Improves Insulin Sensitivity during Aging

Ghrelin via its receptor, the growth hormone secretagogue receptor (GHS-R), increases food intake and adiposity. The tissue-specific functions of GHS-R in peripheral tissues are mostly unknown. We previously reported that while GHS-R expression is very low in white and brown fat of young mice, expression increases during aging. To investigate whether GHS-R has cell-autonomous effects in adipose tissues, we generated aP2-Cre-mediated GHS-R knockdown mice (aP2-Cre/Ghsr^f/f). We studied young (5–6 months) and old (15–17 months) aP2-Cre/Ghsr^f/f mice and their age-matched controls. Interestingly, young aP2-Cre/Ghsr^f/f mice had normal body weight but reduced fat; old mice showed pronounced reductions of both body weight and body fat. Calorimetry analysis revealed that aP2-Cre/Ghsr^f/f mice had normal food intake and locomotor activity at both young and old age; but intriguingly, while energy expenditure was normal at young age, it was significantly increased at old age. Both young and old aP2-Cre/Ghsr^f/f mice exhibited improved insulin sensitivity and glucose tolerance. Importantly, old aP2-Cre/Ghsr^f/f mice maintained higher core body temperature at 4 °C, and showed higher expression of the thermogenic uncoupling protein 1 (UCP1) gene. The ex vivo studies further demonstrated that GHS-R deficient white adipocytes from old mice exhibit increased glucose uptake and lipolysis, promoting lipid mobilization. Despite the fact that the in vivo phenotypes of aP2-Cre/Ghsr^f/f mice may not be exclusively determined by GHS-R knockdown in adipose tissues, our data support that GHS-R has cell-autonomous effects in adipocytes. The anabolic effect of GHS-R in adipocytes is more pronounced in aging, which likely contributes to age-associated obesity and insulin resistance.

Small molecules for fat combustion: targeting thermosensory and satiety signals in the central nervous system

Brown adipose tissue (BAT) dissipates fatty acids as heat to maintain body temperature in cold environments. The existence of BAT and beige cells in human adults supplies a promising weight-reduction therapy. The central thermogenic regulation descends through an excitatory neural pathway from the hypothalamus, medullar and spine towards BAT. This sympathoexcitatory thermogenic circuit is controlled by GABAergic (γ-aminobutyric acid) signaling from the thermoregulatory center in the preoptic area and the satiety center in the ventromedial nucleus of the hypothalamus. This review summarizes recent research progresses in thermogenic regulators targeting thermosensory and satiety signals in the central nervous system, and speculates on their potential as antiobesity agents.

Targeting AgRP neurons to maintain energy balance: Lessons from animal models

The current obesity epidemic is a major worldwide health and economic burden. In the modern environment, an increase in the intake of high-fat and high-sugar foods plays a crucial role in the development of obesity by disrupting the mechanisms governing food intake and energy balance. Food intake and whole-body energy balance are regulated by the central nervous system through a sophisticated neuronal network located mostly in the hypothalamus. In particular, the hypothalamic arcuate nucleus (ARC) is a fundamental center that senses hormonal and nutrient-related signals informing about the energy state of the organism. The ARC contains two small, defined populations of neurons with opposite functions: anorexigenic proopiomelanocortin (POMC)-expressing neurons and orexigenic Agouti-related protein (AgRP)-expressing neurons. AgRP neurons, which also co-produce neuropeptide Y (NPY) and γ-Aminobutyric acid (GABA), are involved in an increase in hunger and a decrease in energy expenditure. In this review, we summarize the key findings from the most common animal models targeting AgRP neurons and the tools used to discern the role of this specific neuronal population in the control of peripheral metabolism, appetite, feeding-related behavior, and other complex behaviors. We also discuss how knowledge gained from these studies has revealed new pathways and key proteins that could be potential therapeutic targets to reduce appetite and food addictions in obesity and other diseases.

The Homeostatic Force of Ghrelin

Ghrelin, a gastric-derived acylated peptide, regulates energy homeostasis by transmitting information about peripheral nutritional status to the brain, and is essential for protecting organisms against famine. Ghrelin operates brain circuits to regulate homeostatic and hedonic feeding. Recent research advances have shed new light on ghrelin's multifaceted roles in cellular homeostasis, which could maintain the internal environment and overcome metaflammation in metabolic organs. Here, we highlight our current understanding of the regulatory mechanisms of the ghrelin system in energy metabolism and cellular homeostasis and its clinical trials. Future studies of ghrelin will further elucidate how the stomach regulates systemic homeostasis.

Effects of meal timing on changes in circulating epinephrine, norepinephrine, and acylated ghrelin concentrations: a pilot study

BACKGROUND

Timing of food intake impacts on metabolic diseases. Few data are available about post-meal changes in epinephrine (E), norepinephrine (NE), and acylated ghrelin (AG) at different times of the day.

SUBJECTS AND METHODS

This randomized cross-over trial investigated E/NE/AG concentrations after identical meals consumed at 0800 or 2000 hours in 20 healthy volunteers, by standardizing diet, exercise, duration of fast, and resting. Participants randomly received the test meal at 0800 or 2000 hours, and vice versa after 1 week. Blood samples were collected before and up to 180-min post-meal, every 30 min, with participants supine, motionless, but awake.

RESULTS

Median E levels increased at 30-60 min, then declined and rose again at 150 min; values at 60 min (19.0 vs. 15.0 ng/l, p = 0.03) and 180 min (25.0 vs. 11.0 ng/l, p < 0.001) were higher after the morning meals. NE rose at 30-60 min and then progressively declined; median values at 60 min (235.3 vs. 206.3 ng/l, p = 0.02) and 120 min (208.8 vs. 142.0 ng/l, p = 0.04) increased more after morning meals. AG progressively declined to increase again at 90 min after meal; median AG area-under-the-curve (AUC) values were lower at morning (7206.8 vs. 8828.3 pg/mL×h). AG-AUC was inversely associated with diet-induced thermogenesis (β = -121.6; 95% CI -201.0 to 42.2; p = 0.009 for each unit increase), while log NE-AUC was inversely associated with log-triglyceride AUC (β = -0.57; 95% CI -0.98 to 0.16; p = 0.015) in a multiple regression model, after multiple adjustments.

CONCLUSIONS

In conclusion, E/NE concentrations were higher after the morning meal, while AG showed an opposite behavior. These data, although requiring confirmation in larger samples, suggest an adjunctive possible mechanism explaining the unfavorable effects of evening eating on metabolic risk.

The Role of Ghrelin and Ghrelin Signaling in Aging

With our aging society, more people hope for a long and healthy life. In recent years, researchers have focused on healthy longevity factors. In particular, calorie restriction delays aging, reduces mortality, and extends life. Ghrelin, which is secreted during fasting, is well known as an orexigenic peptide. Because ghrelin is increased by caloric restriction, ghrelin may play an important role in the mechanism of longevity mediated by calorie restriction. In this review, we will discuss the role of orexigenic peptides with a particular focus on ghrelin. We conclude that the ghrelin-growth hormone secretagogue-R signaling pathway may play an important role in the anti-aging mechanism.

The role of ghrelin-responsive mediobasal hypothalamic neurons in mediating feeding responses to fasting

Objective

Ghrelin is a stomach-derived hormone that affects food intake and regulates blood glucose. The best-characterized actions of ghrelin are mediated by its binding to and activation of the growth hormone secretagogue receptor (GHSR; ghrelin receptor). Adequate examination of the identity, function, and relevance of specific subsets of GHSR-expressing neurons has been hampered by the absence of a suitable Cre recombinase (Cre)-expressing mouse line with which to manipulate gene expression in a targeted fashion within GHSR-expressing neurons. The present study aims to characterize the functional significance and neurocircuitry of GHSR-expressing neurons in the mediobasal hypothalamus (MBH), as they relate to ghrelin-induced food intake and fasting-associated rebound hyperphagia, using a novel mouse line in which Cre expression is controlled by the Ghsr promoter.

Methods

A Ghsr-IRES-Cre mouse line that expresses Cre directed by the Ghsr promoter was generated. The line was validated by comparing Cre activity in reporter mice to the known brain distribution pattern of GHSR. Next, the requirement of MBH GHSR-expressing neuronal activity in mediating food intake in response to administered ghrelin and in response to fasting was assessed after stereotaxic delivery of inhibitory designer receptor exclusively activated by designer drugs (DREADD) virus to the MBH. In a separate cohort of Ghsr-IRES-Cre mice, stereotaxic delivery of stimulatory DREADD virus to the MBH was performed to assess the sufficiency of MBH GHSR-expressing neuronal activity on food intake. Finally, the distribution of MBH GHSR-expressing neuronal axonal projections was assessed in the DREADD virus-injected animals.

Results

The pattern of Cre activity in the Ghsr-IRES-Cre mouse line mostly faithfully reproduced the known GHSR expression pattern. DREADD-assisted inhibition of MBH GHSR neuronal activity robustly suppressed the normal orexigenic response to ghrelin and fasting-associated rebound food intake. DREADD-assisted stimulation of MBH GHSR neuronal activity was sufficient to induce food intake. Axonal projections of GHSR-expressing MBH neurons were observed in a subset of hypothalamic and extra-hypothalamic regions.

Conclusions

These results suggest that 1) activation of GHSR-expressing neurons in the MBH is required for the normal feeding responses following both peripheral administration of ghrelin and fasting, 2) activation of MBH GHSR-expressing neurons is sufficient to induce feeding, and 3) axonal projections to a subset of hypothalamic and/or extra-hypothalamic regions likely mediate these responses. The Ghsr-IRES-Cre line should serve as a valuable tool to further our understanding of the functional significance of ghrelin-responsive/GHSR-expressing neurons and the neuronal circuitry within which they act.

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We generated a novel Ghsr-IRES-Cre knock-in mouse line.

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Cre activity in the line mirrors the known GHSR expression pattern.

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Chemogenetic modulation of neuronal activity reveals a required role of MBH GHSR neurons in rebound food intake after a fast.

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Neuronal projections of mediobasal hypothalamic GHSR neurons are reminiscent of AgRP neuronal projections.

Hunger and Satiety Gauge Reward Sensitivity

Many of the neurocircuits and hormones known to underlie the sensations of hunger and satiety also substantially alter the activity of the dopaminergic reward system. Much interest lies in the ways that hunger, satiety, and reward tie together, as the epidemic of obesity seems tied to the recent development and mass availability of highly palatable foods. In this review, we will first discuss the basic neurocircuitry of the midbrain and basal forebrain reward system. We will elaborate how several important mediators of hunger-the agouti-related protein neurons of the arcuate nucleus, the lateral hypothalamic nucleus, and ghrelin-enhance the sensitivity of the dopaminergic reward system. Then, we will elaborate how mediators of satiety-the nucleus tractus solitarius, pro-opiomelanocortin neurons of the arcuate nucleus, and its peripheral hormonal influences such as leptin-reduce the reward system sensitivity. We hope to provide a template by which future research may identify the ways in which highly rewarding foods bypass this balanced system to produce excessive food consumption.